1. Field of the Invention
The present invention relates to a pressure intensifying apparatus for a hydraulic cylinder, and, more particularly, to an apparatus which is adapted for use for a core driving cylinder in a die cast machine or an injection molding machine and deals with the phenomenon that a hydraulic fluid is compressed to push the core back at the time of casting or applying pressure.
2. Description of the Related Art
A die cast machine employs a casting scheme of injecting a molten metal or semimolten alloy to which a molding pressure is applied by an injection cylinder into a cavity whose shape corresponds to the shape of a product and which is formed by assembling a movable mold and the core to a fixed mold, via a runner. The pressure molding can provide castings of relatively complicated shapes with high precision.
The conventional core driving hydraulic cylinders usually use a hydraulic circuit as shown in FIG. 10.
Referring to the diagram, a hydraulic cylinder 200 comprises a cylinder tube 201, a rod cover 202, a head cover 203, a piston 204 and a rod 205. A 4-port, 3-position switching valve 210 allows the supply and discharge of a hydraulic fluid to and from a rod-side port 206 and a head-side port 207 via meter-in circuits 208 and 209. The hydraulic cylinder 200 is designed in such a manner that a core 211 attached to the piston rod 204, 205 is fitted in and pulled out from a predetermined hole or the like, which is formed in a mold, in the advancing/reversing step. A pilot check valve 212 is disposed between the head-side port 207 of the hydraulic cylinder 200 and the meter-in circuit 209, and its pilot pressure comes from the rod-side port 206.
With the structures of the hydraulic cylinder 200 and the hydraulic circuit, first, the switching valve 210 is set to the state shown in FIG. 10, and then the hydraulic fluid is supplied from the head-side port 207 to move the piston rod 204, 205 forward from the backward limit. This causes the core 211 to enter the mold to a predetermined position. When the core 211 reaches the insertion limit, the pressure of a head-side cylinder chamber 213 of the hydraulic cylinder 200 becomes the rated pressure of a hydraulic pump 214. With the pressure balanced, the piston 204 stops, closing the open pilot check valve 212.
As a result, the head-side cylinder chamber 213 of the hydraulic cylinder 200 is sealed tightly and the piston rod 204, 205 is locked. Then, a molten metal is injected into the cavity under high pressure (molding pressure) and is left in that state for solidification, which completes casting.
When casting is completed, the switching valve 210 is switched to the opposite supply and discharge position, and the hydraulic fluid is supplied from the rod-side port 206 to move the piston rod 204, 205 backward. In this case, the pilot pressure acts on the pilot check valve 212 to open the closed valve 212, and the piston rod 204, 205 returns to the original backward limit.
Thereafter, the cavity is opened for removal of the casting.
When a molten metal is injected into the cavity with the core 211 pressed into the mold to its positional limit, the molding pressure of the molten metal is applied to the core 211 and the force FL equivalent to the molding pressure xc3x97 the pressure receiving area acts on the core 211. This applies a large push-back load on the piston rod 204, 205 of the hydraulic cylinder 200.
In this case, as the pilot check valve 212 is closed, the piston rod 204, 205 is held locked. The generation of such a push-back load normally raise no problems. Because the load is generally intensive reactive force, which is several times the rated pressure of the hydraulic pump 214, however, a very large pressure is applied to the hydraulic fluid in the head-side cylinder chamber 213.
Specifically, the pressure of the hydraulic fluid becomes PLa=FL/Ah where FL is the push-back load on the core 211 and Ah is the pressure receiving area of the piston 204. Given that the rated pressure of the hydraulic pump 214 is Pp, the pressure of the hydraulic fluid increases by xcex94Pa=PLaxe2x88x92Pp after the core 211 is moved forward to the positional limit and the pilot check valve 212 is closed.
The hydraulic fluid is not a complete incompressive fluid, but has a compressibility, very small though it is.
Accordingly, given that the volume of the head-side cylinder chamber 213 is Va and the compressibility of the hydraulic fluid is xcex2, the hydraulic fluid is compressed by xcex94V=xcex94Paxc3x97Vaxc3x97xcex2, with the result that the piston rod 204, 205 is pushed back by (xcex94V/Ah).
The push-back quantity is naturally decided depending on the conditions of the respective elements relating to the aforementioned equation. In actuality, the quantity is of such magnitude that it may be almost ignored with respect to the process length of the normal hydraulic cylinder since the compressibility of the hydraulic fluid: xcex2 is low.
However, in recent years, high accuracy of dimension has been required of die cast products, and products having no secondary operation needed are often demanded of auto-parts, parts for electric equipment, and the like, and an error in size caused by the aforementioned push-back cannot be ignored in some cases.
As measures against the above problem, there is a case in which a mold and the like are designed in consideration of the above-mentioned error beforehand. The specification of the hydraulic cylinder adapted to the die cast machine is not uniformly provided, a case in which the change in the specification is made must be assumed, and the above measures are unrealistic in view of the point that the aforementioned error changes depending on the condition of the hydraulic cylinder.
It is an object of the present invention is to provide an apparatus wherein a small-sized pressure intensifying cylinder designed rationally is additionally provided to a working cylinder is added and the pressure intensifying cylinder is controlled adaptively in an advancing step or reversing step, whereby solving the aforementioned problem.
The first invention relates to a pressure intensifying apparatus for a hydraulic cylinder comprising: a working cylinder; a pressure intensifying cylinder for allowing a pressure intensifying chamber to communicate with a head-side cylinder chamber of the working cylinder; a switching valve for controlling supply and discharge of a hydraulic fluid with respect to a head-side port and a rod-side port of the working cylinder; a first pilot check valve which is disposed in a head-side supply and discharge circuit for connecting the head-side port of the working cylinder to the switching valve and whose pilot pressure is a pressure of a rod-side supply and discharge circuit for connecting the rod-side port of the working cylinder to the switching valve; a check-valve equipped sequence valve which is disposed in a connection circuit for connecting a drive chamber side of the pressure intensifying cylinder to a portion of the head-side supply and discharge circuit closer to the switching valve than the first pilot check valve, and switches from a closed state to an open state with a rated pressure of a hydraulic pump for supplying the hydraulic fluid to the head-side supply and discharge circuit via the switching valve; and a second pilot check valve whose pilot pressure is the pressure of the rod-side supply and discharge circuit, the working cylinder, the pressure intensifying cylinder, the switching valve, the first pilot check valve, the check-valve equipped sequence valve and the second pilot check valve being connected in series in such a way as to satisfy K greater than PLa/Pp where K is a pressure intensifying ratio of the pressure intensifying cylinder, Pp is the rated pressure of the hydraulic pump and PLa is a pressure in the head-side cylinder chamber when a maximum load occurring after Pp is balanced with a load acting on a piston rod of the working cylinder in a piston-rod advancing step of the working cylinder is applied, and xcex94V=(PLaxe2x88x92Pp)xc3x97Vaxc3x97xcex2 where xcex94V is a maximum discharge volume of the pressure intensifying chamber of the pressure intensifying cylinder, Va is a volume of the head-side cylinder chamber when balance is held and xcex2 is a compressibility of the hydraulic fluid.
According to this invention, when the rated pressure of the hydraulic pump and the load acting on the piston rod are balanced with each other in the advancing step of the piston rod of the working cylinder, the first pilot check valve is switched from the open state to the close state, and the check-valve equipped sequence valve and the second pilot check valve is switched from the close state to the open state, so that the pressure intensifying cylinder is automatically advanced.
The pressure intensifying ratio of the pressure intensifying cylinder: K is set to a value that gives the advancing force, which exceeds the maximum load of the push-back acting on the piston rod after the balance is held, to the piston rod based on the condition of K greater than PLa/Pp. Also, the maximum discharge volume of the pressure intensifying chamber: xcex94V is set to a value that compensates for the compression quantity of the hydraulic fluid of the head-side cylinder chamber based on the condition of xcex94Vxe2x88x92(PLbxe2x88x92Pp)xc3x97Vbxc3x97xcex2.
The reason why (PLaxe2x88x92Pp) is given is as follows:
More specifically, since the hydraulic fluid is compressed with Pp in advance, a necessary compensation quantity is only the compression quantity corresponding to (PLaxe2x88x92Pp), which is equivalent to an increase in pressure.
Accordingly, after the state where balance is held, only the hydraulic fluid, which is equivalent to the compression quantity, is supplied to the head-side cylinder chamber in such a way that the pressure of the cylinder chamber is automatically increased to be higher than PLa. For this reason, the piston rod is locked at a predetermined position without being pushed back by the compression of the hydraulic fluid.
According to this invention, the supply and discharge control for the working cylinder and the pressure intensifying cylinder can be performed by a single switch valve.
The above pressure intensifying apparatus relates to the case in which the working cylinder receives the load in the piston rod advancing step. In the case of opposing to the maximum load occurring in the reversing step, the pressure intensifying chamber of the pressure intensifying cylinder is made to communicate with the rod-side cylinder chamber of the working cylinder, and the other conditions relating to the arrangement of hydraulic circuit and the pressure intensifying cylinder may be provided in reverse at the head side and the rod side.
The second invention relates to a pressure intensifying apparatus for a hydraulic cylinder comprising: a working cylinder; a pressure intensifying cylinder for allowing a pressure intensifying chamber to communicate with a head-side cylinder chamber of the working cylinder; a switching valve for controlling supply and discharge of a hydraulic fluid with respect to a head-side port and a rod-side port of the working cylinder; a pilot check valve which is disposed in a head-side supply and discharge circuit for connecting the head-side port of the working cylinder to the switching valve and whose pilot pressure is a pressure of a rod-side supply and discharge circuit for connecting the rod-side port of the working cylinder to the switching valve; a pressure sensor for detecting a pressure in the head-side cylinder chamber of the working cylinder; and
pressure intensification control means for controlling supply and discharge of the hydraulic fluid to and from a drive chamber of the pressure intensifying cylinder using a detection signal from the pressure sensor in a piston-rod advancing step of the working cylinder, wherein K greater than PLa/Pp is satisfied where K is a pressure intensifying ratio of the pressure intensifying cylinder, Pp is a rated pressure of a hydraulic pump and PLa is a pressure in the head-side cylinder chamber when a maximum load occurring after Pp is balanced with a load acting on a piston rod of the working cylinder in a piston-rod advancing step of the working cylinder is applied, and xcex94Vxe2x89xa7(PLaxe2x88x92Pp)xc3x97Vaxc3x97xcex2 where xcex94V is a maximum discharge volume of the pressure intensifying chamber of the pressure intensifying cylinder, Va is a volume of the head-side cylinder chamber when balance is held and xcex2 is a compressibility of the hydraulic fluid, and the pressure intensification control means supplies the hydraulic fluid to the drive chamber of the pressure intensifying cylinder when Sr greater than Ssxe2x89xa7Sp where Ss is a level of the detection signal from the pressure sensor, Sp is a level of an output signal corresponding to Pp and Sr is a level of an output signal corresponding to PLa, and discharges the hydraulic fluid from the drive chamber of the pressure intensifying cylinder when Ss greater than Sr.
According to this invention, the pressure of the head-side cylinder chamber is detected by the pressure sensor without using the check-valve equipped sequence valve unlike the first invention. When the detected pressure Ps is PLa greater than Psxe2x89xa7Pp, the pressure intensifying cylinder pressurizes the head-side cylinder chamber of the working cylinder, and when it is Ps greater than PLa, the pressure intensifying cylinder depressurizes the head-side cylinder chamber.
Namely, when the pressure of the head-side cylinder chamber of the working cylinder is more than Pp, the pressure is automatically increased to keep PLa.
Since adaptive control is performed as measuring the pressure of the head-side cylinder chamber in actual, the condition of the pressure intensifying ratio of the pressure intensifying cylinder: K is the same as that of the first invention, but the maximum discharge volume of the pressure intensifying chamber: xcex94V may be more than [(PLaxe2x88x92Pp)xc3x97Va xc3x97xcex2 without having to be set fixedly.
Namely, in this invention, discharge and supply control for the working cylinder and pressure intensifying cylinder must be individually executed. However, in the case of xcex94V greater than (PLaxe2x88x92Pp)xc3x97Vaxc3x97xcex2, this invention has an advantage in which the pressure intensifying cylinder having a common specification can deal with the wide range of the specification of the working cylinder.
In the case of opposing to the maximum load occurring in the reversing step, the pressure intensifying chamber of the pressure intensifying cylinder is made to communicate with the rod-side cylinder chamber of the working cylinder, and the pressure of the rod-side cylinder chamber is measured by the pressure sensor and the pilot check valve may be provided in reverse at the head side and the rod side.
The third invention relates to a pressure intensifying apparatus for a hydraulic cylinder comprising: a working cylinder; a pressure intensifying cylinder for allowing a pressure intensifying chamber to communicate with a head-side cylinder chamber of the working cylinder; a switching valve for controlling supply and discharge of a hydraulic fluid with respect to a head-side port and a rod-side port of the working cylinder; a pilot check valve which is disposed in a head-side supply and discharge circuit for connecting the head-side port of the working cylinder to the switching valve and whose pilot pressure is a pressure of a rod-side supply and discharge circuit for connecting the rod-side port of the working cylinder to the switching valve; position detection means for detecting when a piston rod of the working cylinder reaches a known position of the piston rod where a rated pressure of a hydraulic pump is balanced with a load acting on the piston rod of the working cylinder in a piston-rod advancing step of the working cylinder; and pressure intensification control means for starting supplying the hydraulic fluid to a drive chamber of the pressure intensifying cylinder based on a detection signal from the position detection means in the piston-rod advancing step of the working cylinder, wherein K greater than PLa/Pp is satisfied where K is a pressure intensifying ratio of the pressure intensifying cylinder, Pp is the rated pressure of the hydraulic pump and PLa is a pressure in the head-side cylinder chamber when a maximum load occurring after the position detection means makes that detection is applied, and xcex94V=(PLaxe2x88x92Pp)xc3x97Vaxc3x97xcex2 where xcex94V is a maximum discharge volume of the pressure intensifying chamber of the pressure intensifying cylinder, Va is a volume of the head-side cylinder chamber when the position detection means makes the detection and xcex2 is a compressibility of the hydraulic fluid.
The second invention uses the pressure sensor, whereas this invention uses position detection means for detecting the position of the piston rod of the working cylinder when reaching a predetermined position.
Generally, in the case where the pressure intensifying apparatus is adapted to the hydraulic cylinder, the position of the piston rod where the working cylinder needs pressure intensifying is experientially known in many cases. This makes it possible to perform the start of driving the pressure intensifying cylinder based on position detecting information.
Various kinds of prior art such as means using a limit switch, means using a magnetic sensor or an inductance sensor can be used as position detecting means.
It is noted that the conditions of the pressure intensifying cylinder are the same as those of the first invention since this case aims to detect only the timing at when the pressure intensifying cylinder is started to drive.
In the case of opposing to the maximum load occurring in the reversing step, the pressure intensifying chamber of the pressure intensifying cylinder is made to communicate with the rod-side cylinder chamber of the working cylinder, and the pilot check valve may be provided in reverse at the head side and the rod side.